The present invention relates to the formulation of catalysts and in particular, to the binding of active catalytic molecules to polymeric supports.
The concept of binding active catalytic molecules to polymeric or other supports initially was developed by biochemists who used these systems for enzymatic purposes and as a method for conveniently separating or recovering the catalytic material from the reaction mixture. More recently, as will be described, these systems have been the object of a number of studies which, in particular, recognize the advantage gained by the inherent simplification and ease of the catalyst recovery and separation.
The polymer-bound catalysts of the present invention can be identified as heterogeneous systems as contrasted with the more common and increasingly important homogeneous catalytic systems. For the most part, however, the present systems are based upon the homogeneous systems. The distinction between the two lies principally in the fact that, in the homogeneous systems, the active catalytic material is only weakly associated with or deposited on its support material whereas, in the heterogeneous systems, the material is securely anchored by means of a strong chemical bond. Also, the homogeneous catalysts usually are soluble in the reaction mixture. In some situations, the distinction becomes difficult. However, insofar as the present invention is concerned, the term `heterogeneous` is applied to polymer-bound systems in which the catalytic material is chemically bonded with sufficient strength to permit ready separation of the catalyzed product from catalytic material without appreciable catalyst loss or degradation of its activity. In the homogeneous systems, the dissociation and/or solubility of the catalytic material renders recovery of the material difficult and also presents a problem in separating the product itself. Consequently, redeposition of the material is required for recycling as opposed to the simplified filtering and recycling procedure provided by the heterogeneous catalysts.
Previous work by others has produced some interesting findings. For example, in the later 1960's several catalytic systems based upon homogeneous systems were described in the literature. One of the first extensively studied polymer bound homogeneous systems was that of the polystyrene-bound analog of Wilkinson's catalyst described in the following reports:
Grubbs, R. H., and Kroll, L. C., J. Amer. Chem. Soc. 93, 3062 (1971) PA1 Grubbs, R. H. Kroll, L. C., and Sweet, E. J., J. Macromol - Sci. Chem. A7(5): (1973) PA1 Collman, J. P., et al., J. Amer. Chem. Soc., 94, 1789 (1972) PA1 Pittman, C. U. and Hanes, R. "Frontiers in Organometallic Chemistry" Ann, NY Acad. Sci., 239, 76 (1974) PA1 Evans, D. Osborn, J. A. and Wilkinson, G., and J. Chem. Soc (A), 3133 (1968) PA1 Pittman, C. U., Smith, L. R., and Hanes, R. J. Amer. Chem. Soc. 97, 1742 (1975) PA1 Allum, K. G., Hancock, R. D., Howell, I.V. Pitkethly, R. C. and Robinson, P. J., J. Organometal Chem., 87 189 (1975) PA1 Allum, K. G., Hancock, R. D., Howell, I.V., McKenzie, S. Pitkethly, R. C. Robinson, P. J. Organometal, Chem. 87, 203 (1975)
Wilkinson's catalyst was chosen because of a great deal of work that had been published on its catalytic reaction mechanisms. It was found that the complex appeared to function in the polymer bead as it would in solution for the hydrogenation of olefins. However, the activity was decreased by the binding with the lowered activity attributed to the slow diffusion of the olefin through the polystyrene matrix. This property, however, allowed a good deal of selectivity on the part of the catalyst and the catalyst was easily separated and recycled.
Similar studies were conducted with RhH(CO)(PPh.sub.3).sub.3 in hydroformylation reactions. This polymer-bound catalyst retained catalytic activity through several successive reaction-separation-reaction cycles and no loss of rhodium was observed. See:
A variety of polymer-attached transition metal catalytic systems also have been synthesized and reported in the following publications:
These systems appear to be analogous in behavior to the homogeneous catalysts from which they were derived.
Besides varying the active complex, the polymeric support also can be designed to provide desired properties and a wide range of supports have been utilized. The two Organometal Chem. publications cited supra provide examples of this work.
As far as is known, polymer-bound metallocarborane catalyst systems have not been synthesized and described in the literature. Some systems have combined a polymer and carborane but the chemical attachment was through a phosphine-to-metal bond rather than through the direct bonding of the metallocarborane to the polymer which characterizes the present invention. In the reactions the phosphine-to-metal bond breaks down with the result that some metal catalyst is lost in the mixture so that the recovered catalyst loses some of its activity.